1. Field of the Invention
The invention relates to a recursive surface active wave filter having at least three tracks connected in series and/or parallel.
2. Description of the Related Art
Surface-active wave filters, also referred to as SAW filters, can, for example, be employed as intermediate frequency filters in the reception part of a mobile telephone. These filters must meet various demands, including providing an adequately broad pass-band, a high edge steepness, and a best possible selection on the smallest possible chip area.
A maximum edge steepness can be achieved with a filter having lengthened pulse response since the edge steepness is directly dependent on the length of the pulse response. In a transverse filter, the length of the pulse response is directly determined from the length of the interdigital transducers utilized for the electro-acoustic conversion. Given the specification-conditioned, maximally allowed chip length, it is therefore not possible to meet the specifications demanded for mobile telephones with a transverse filter. For this reason, technologies have prevailed in mobile telephones that use a folded propagation path of the surface-active wave. Resonance chambers are created on the surface of the filter with the assistance of reflective structures, these resonance chambers serving the purpose of achieving a greater edge steepness with the resultant lengthened pulse response. A given length of the pulse response can be utilized for achieving the most beneficial compromise between pass-band behavior (i.e., the shape of the pass region or passband) and the edge steepness and the selection (i.e., the suppression of signal in the stop band). When the first two demands are to be met by a transducer having a minimal length, then its selection is necessarily inadequate.
Surface-active wave filters that exhibit the desired transmission behavior have previously been realized with various methods. The simplest solution that, however, cannot be utilized in mobile radiotelephony is to lengthen the filter and, thus, the chip on which the filter is realized to the extent that the desired transmission behavior is achieved. A more complex and, thus, improved transmission behavior can be achieved with a longer filter. This, however, works against the desired miniaturization of the filters.
It is also possible to increase the reflections in the surface-active wave structures of the transducers or reflectors. Stronger resonances with which the duration of the pulse response is lengthened are thus generated. Given crystal substrates, however, the reflection is highly dependent on the relative layer thickness of the metallization. The necessary, intense reflection can only be achieved with a high metallization layer thickness, which, however, leads to an increased sensitivity to technology-conditioned fabrication scatter in the manufacture of the filters.
Another possibility is comprised in realizing a filter with the desired transmission behavior in a plurality of acoustic coils in which identical transducers are connected in parallel or not in parallel. The frequency-dependent reflection at the acoustic transducer ports is thereby utilized in order to additionally influence the transfer function. However, no additional degrees of freedom in the design of these filters are achieved with such a configuration. Given identical filter properties, only the demands made of the individual acoustic track can be lowered, for example with respect to the selection.
Another possibility is to use a SAW filter with exactly two different acoustic tracks in which respectively two interdigital transducers are connected in parallel both at the filter input as well as at the filter output. A greater number of degrees of freedom in the design of the filter are achieved by this. In particular, the selection of the filter is improved, this being clearly greater for the overall filter than for an individual track or for a sub-filter. Improvements in the selection of the overall filter compared to the selection of the individual tracks, however, are only achieved when the transfer functions of the two individual tracks in the stop band are of the same amount and opposite phase, since only then can unwanted signals in the stop band be quenched.